This invention concerns a system for applying axial tension to a blanket in a shoe press of a papermaking machine.
In a shoe press, an elongate shoe having a concave surface forms a nip with a backing roll. This lengthens the duration of contact with a paper web passing through the nip to allow more water to pass from the web into one or more supporting felts, as required in modern high speed presses where the paper web may be traveling at something in the order of 15 m/sec.
In order for the paper web to move through the nip without frictional resistance, a blanket in the form of a seamless tube is passed over the shoe. The blanket is lubricated and cooled by a supply of oil or other lubricant which is forced between the blanket and the shoe. To prevent leakage of lubricant, which could contaminate the paper web and surrounding equipment, the blanket is sealed at each end to a respective head. The heads, referred to as front and back heads, each have an outer portion (termed an outer head) which is circular and is mounted rotation on a journal of a support beam, which also supports the shoe and a hydraulic piston which urges the shoe towards the backing roll.
As the blanket is caused to move through the nip, by virtue of frictional engagement with the paper web, any supporting felt, and the backing roll, it assumes a dimpled shape (rather like an apple) in cross section in conformity with the concave shoe. However, the blanket extends beyond each end of the shoe to attach to the circular outer heads. The transition from the dimpled shape to the circular shape means that in the region adjacent each end the blanket undergoes repeated changes in curvature, resulting in wear and eventual failure. This can be a significant source of maintenance down time and contributor to the cost of the papermaking process.
The motion of the blanket through the nip also results in gradual lengthening of the blanket in the cross machine direction owing to its continual compression. In this respect, over a period of about 6 months, it can stretch or lengthen by something of order of 15 to 20 cm. To some extent this can be accommodated by a indexing system comprising hydraulic piston and cylinder assemblies which are mounted between the support beam and each head to apply tension to the heads. Periodically, however, the blanket has to be trimmed, which involves releasing it from the heads and refixing. This may result in 6 to 8 hours downtime.
The indexing system serves the further purpose, of course, of keeping the blanket shape stable as it rotates during operation of the press, preventing creasing and twisting as it passes through the nip.
Hitherto, as described in U.S. Pat. Nos. 5,775,564, 5,643,416, 5,733,415, and 5,900,118, the position of one head, attached to the end of the blanket, has been fixed, by specific locking, by control of the position of its hydraulic cylinders, or by abutment against stop means, whilst the other head, attached to the other end of the blanket, is allowed to float. It may be held under substantially constant tension by means of a hydraulic cylinder, but its position is not fixed, so it can move axially to take up slack as the blanket stretches or lengthens. Periodically, the hydraulic systems attached to the front and back heads are reversed, so the floating head then becomes fixed and the formerly fixed head floats. This changeover may take place every few days or weeks. The wear at the fixed end is always greater than that at floating end because the region of the blanket subject to wear tends not to move so much at the fixed end.
The invention provides a method of applying axial tension to a blanket in a shoe press of a papermaking machine which is characterized in that both heads are permanently floating and axial pressure applied thereto is controlled such that the blanket maintains a substantially central position on the press and elongation of the blanket during operation of the press is taken up substantially equally by the first and second heads. Thus, neither head is fixed in position at any time.
Furthermore, the apparatus of the shoe press includes respective fluid actuated piston and cylinder assemblies mounted inside the blanket and operative to apply axial pressure to the first and second heads, thereby to tension the blanket, characterized in that the respective fluid assemblies include respective displacement sensors whereby the positions of the heads, and thereby of the blanket, relative to the press can be determined and monitored, and in that means are provided whereby automatic adjustments are then made to the positions of the heads to maintain the blanket in a substantially central position on the press. The heads are limited in travel by the stroke of their respective cylinders. No other mechanical means of limiting the travel of the heads needs to be provided.
An object of the invention is to provide an improved system of head indexing in a shoe press of the type described above, whereby the life of the blanket may be increased.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon a review of the following detailed description of the preferred embodiments and the accompanying drawings.